1. Field of the Invention
The present invention relates to an antenna apparatus for use in a radio communication apparatus which is controlled so as to achieve high-speed communication with increased communication capacity as well as keep high communication quality in mobile communication through a mobile telephone or the like. The present invention particularly relates to a MIMO antenna apparatus controlling the number of streams and the modulation and demodulation method, and a radio communication apparatus provided with the MIMO antenna apparatus.
2. Description of the Related Art
There has been known an antenna apparatus using MIMO (Multi-Input Multi-Output) technique for simultaneously transmitting and/or receiving radio signals on a plurality of channels using a plurality of antenna elements, and such antenna apparatus includes, for example, a MIMO antenna apparatus disclosed in the Japanese patent laid-open publication No. 2004-266586 (referred to as “a first prior art document” hereinafter).
A mobile communication system with the MIMO antenna apparatus of the first prior art document is configured in its sender side as follows: a transmission signal is channel-encoded into number M of signals by a channel encoder, these number M of signals are modulated into number M of complex modulated signals (modulated symbols) by number M of modulators respectively, these number M of complex modulated signals are then multiplied to a complex matrix consisting of number M*L of complex coefficients by a complex matrix operation unit such that number L of complex signals are generated, and these number L of generated complex signals are transmitted through number L of transmitting antenna elements respectively. In this configuration, the complex matrix operation unit performs a matrix operation so as to weight the number M of modulated signals with different complex weight coefficients respectively, and this reads to achieving the beam forming on the radio signals transmitted from the number L of transmitting antenna elements respectively. Thus, the mobile communication system of the first prior art document is intended to attain transmission diversity effect, as well as the beam forming for suppressing interference waves. Furthermore, the invention of the first prior art document provides a data transmission method in which the modulation method and transmission rate are appropriately controlled even under a dynamically variable propagation channel environment, by virtue of the diversity effect. Moreover, the mobile communication system equipped with the MIMO antenna apparatus of the first prior art document is configured in its receiver side as follows: number N of received signals that have been received through number N of receiving antenna elements are multiplied to a complex matrix consisting of N*M complex coefficients by a MIMO demodulator such that number M of complex signals (received symbols) are generated, the number M of complex signals are demodulated into number M of demodulated signals by number M of demodulators respectively, and then, the number M of demodulated signals are channel-decoded into a received data by a channel decoder. In this configuration of the receiver side MIMO demodulator, the influences of noises and interferences can be minimized by employing the MMSE (Minimum Mean Square Error) algorithm for reducing interferences.
Thus, according to the first prior art document, the mobile communication system equipped with the MIMO antenna apparatus can be provided, such that at the sender side of the mobile communication system, a high-speed data communication through multiplexed channels can be achieved by multiplying the number M of modulated signals to the complex matrix consisting of the number M*L of elements so that the number L of complex signals are generated and transmitting the number L of complex signals through the number L of transmitting antenna elements respectively, and at the receiver side of the mobile communication system, the limit of interference can be extended by reducing the interferences in the MIMO demodulator.
Moreover, there has been known a prior art MIMO antenna apparatus having transversal filters, and such antenna apparatus includes, for example, a MIMO antenna apparatus disclosed in the Japanese patent laid-open publication No. 2005-065197 (referred to as “a second prior art document” hereinafter).
A MIMO-OFDM receiver equipped with the MIMO antenna apparatus of the second prior art document is configured to eliminate interference waves by transversal filters each provided for each one of a plurality of receiving antenna elements, and then to perform MIMO demodulation. In this way, the radio signals can be received by using MIMO, even in an environment including the interference waves. Therefore, according to the technique of the second prior art document, the MIMO-OFDM receiver can be provided, which can suppress the interferences, compensate for deterioration of accuracy in timing recovery and channel estimation, and achieve high-speed signal transmission in any case. Thus, according to the second prior art document, the MIMO-OFDM receiver equipped with the MIMO antenna apparatus is provided, which can receive radio signals by using MIMO and achieve the high-speed radio transmission even in the environment including interference waves, by eliminating the interference waves by means of the transversal filters each provided for each one of the receiving antenna elements and then performing the MIMO demodulation,
However, the prior art MIMO antenna apparatus of the first prior art document has the following problems. The first prior art document discloses the MIMO antenna apparatus which can attain the transmit diversity effect as well as achieve the beam forming for suppressing the interference waves, by providing the sender side with the number M of modulators and the number L of transmitting antenna elements, in order to increase the data transmission rate as high as possible. However, since the MIMO antenna apparatus of the first prior art document has many transmitting antenna elements, it is quite difficult to mount a number of antenna elements on a small-sized apparatus with a size of one wavelength or less, such as a mobile telephone. Moreover, even if the MIMO antenna apparatus of the first prior art document is employed at a base station of the mobile telephone system, the cost increases and the control becomes more complicated in response to an increase in the number of antenna elements.
Meanwhile, the prior art MIMO antenna apparatus of the second prior art document using the plurality of transversal filters has the following problems. This prior art MIMO antenna apparatus can suppress the interference waves since the transversal filters are provided for the respective receiving antenna elements, however, the size of receiver circuit becomes disadvantageously larger. In other words, it is impossible to configure this prior art MIMO antenna apparatus into a small-sized form, and to use this prior art MIMO antenna apparatus in a battery-operated mobile radio apparatus.